Oil prices have surged more than 50% since the summer—a sign investors are reassessing what was once the biggest risk in the market: U.S. shale.

For more than three years, the ability of shale producers to rapidly dial production up or down has haunted the oil market, cutting off nascent rallies and keeping prices trapped at around $50 a barrel or lower. Any move above that level stoked fears that producers would flood the market with oil again, causing dramatic reversals in prices.